Interested in Russia Investigation?

Add Russia Investigation as an interest to stay up to date on the latest Russia Investigation news, video, and analysis from ABC News.

Those interviewed included longtime Trump friend and confidant Thomas Barrack, who oversaw the fundraising effort, as well as individuals familiar with the massive inaugural fund, according to sources with direct knowledge.

Barrack, a real estate investor, has long been described as a Trump “whisperer” whose close friendship with the president landed him a prime appearance during the GOP convention the night Trump accepted his party’s nomination.

The billionaire runs a fund with hundreds of millions in real estate and private equity holdings in the Middle East. Barrack oversaw the largest inaugural fundraising effort in U.S. history, bringing in $107 million – more than double what President Barack Obama raised for his first swearing-in festivities.

According to a source who has sat with the Mueller team for interviews in recent weeks, the special counsel is examining donors who have either business or personal connections in Russia, Saudi Arabia, the United Arab Emirates and Qatar. Several donors with those ties contributed large sums to the non-profit fundraising entity – gifts that topped out at $1 million dollars, according to public records.

Mandel Ngan/AFP/Getty Images

President Donald Trump takes the oath of office during his swearing-in ceremony on January 20, 2017 at the US Capitol in Washington, DC.

Special counsel investigators have also asked witnesses about specific inauguration donors, including American businessmen Leonard Blavatnik, and Andrew Intrater, according to sources familiar with the Mueller sessions.

Neither has been accused of any wrongdoing.

Blavatnik is a billionaire with dual U.S. and British citizenship who has extensive business ties in Russia. Blavatnik gave $1 million to the inaugural fund through his company, Access Industries, according to FEC records. Companies are prohibited from giving donations to political candidates, however, donations to inaugural committees are not considered donations to candidates.

A spokesman for Blavatnik did not respond to questions from ABC News.

Intrater, an American relative and business associate of Russian billionaire Viktor Vekselberg, runs a U.S. company with deep ties to Vekselberg’s Russia-based global conglomerate, Renova Group. Renova was recently sanctioned by the U.S. Treasury Department.

Intrater serves as the CEO of Columbus Nova, an investment company based in New York. FEC records show Intrater made a $250,000 donation to the Trump inauguration committee in early January 2017.

Following the donation, he and Vekselberg attended Trump’s inauguration, according to published reports. Also in 2017, Intrater made two more donations to the Trump orbit including a $35,000 to Trump Victory, the joint fundraising committee for the Trump campaign and Republican party, plus a separate $29,600 donation directly to the Republican National Committee, according to FEC records.

Intrater has not responded to inquiries from ABC News.

Samuel Corum/Anadolu Agency via Getty Images

President-elect Donald Trump and Vice President-elect Mike Pence during the 58th U.S. Presidential Inauguration in Washington, D.C.on January 20, 2017.

Vekselberg, a Russian national, was questioned by the Mueller team after stepping off his private jet at a New York-area airport, The New York Times reported earlier this month. Vekselberg has also been in the headlines this week after the attorney for adult film star Stormy Daniels, Michael Avenatti, alleged that Veksleberg directed payments to a company formed by Cohen’s totaling $500,000.

The alleged payments, made by Columbus Nova — the firm led by Intrater — were listed in a seven-page document prepared by the law firm representing Stormy Daniels.

Daniels alleges she had a sexual encounter with Trump in 2006.

The document outlines what it claims are $4.4 million in payments to a corporate entity set up by Cohen shortly before the 2016 elections. The payments allegedly come from both obscure companies and well-known brands — foreign and domestic — including AT&T. Several of those companies have extensive business before the U.S. government.

The alleged payments continued until earlier this year.

Columbus Nova said in a statement earlier this week that the firm’s work with Cohen was not improper.

“Columbus Nova is a management company solely owned and controlled by Americans. After the inauguration, the firm hired Michael Cohen as a business consultant regarding potential sources of capital and potential investments in real estate and other ventures,” Columbus Nova’s attorney Richard Owens said in the statement. “Reports today that Viktor Vekselberg used Columbus Nova as a conduit for payments to Michael Cohen are false. The claim that Viktor Vekselberg was involved or provided any funding for Columbus Nova’s engagement of Michael Cohen is patently untrue. Neither Viktor Vekselberg nor anyone else other than Columbus Nova’s owners were involved in the decision to hire Cohen or provided funding for his engagement.”

Why these individuals would be of interest to the Mueller team remains unclear. The expanded probe sheds light on a possible growing area of interest to the Mueller investigation.

So far, the Mueller probe has led to nearly 20 indictments including plea deals for cooperation for two Trump loyalist – former National Security Advisor Michael Flynn and Deputy Campaign Manager Rick Gates.